This invention relates to a variable speed drive of the hydromechanical type in which the drive ratio is varied by means of fluid pump-motor units.
Hydromechanical transmissions wherein two power paths are combined by a planetary gear unit to obtain a drive ratio that is varied through a hydrostatic pump-motor assembly in one of the power paths are well known. Examples of such hydromechanical transmissions are dis-closed in U.S. Pat. Nos. 3,090,253, 3,736,813 and 3,733,931.
The foregoing type of hydromechanical drive is often used in combination with a plural change speed gear box of the shiftable type because of the relatively small variation in drive ratio associated with the hydromechanical drive and the relatively low torque capacity of the fluid pump-motor units. The shift type of change speed transmission will therefore provide a plurality of speed range steps while the fluid drive units are operative to effect an infinite variation in drive ratio within each speed range. Transmission combinations of the foregoing type are particularly suited for heavy duty vehicles such as earth moving vehicles and agricultural tractors.
A serious problem arises in heavy duty installations employing the aforementioned types of transmission assemblies because of a destructive overspeed condition that occurs whenever a relatively large input torque is suddenly applied as would occur, for example, during starting or sudden clutch engagement. The overspeed condition arises because of the low numerical drive ratio of the planetary gear unit and is accompanied by excessive torque loading of the fluid drive unit. It is therefore an important object of the present invention to provide a hydromechanical transmission that will automatically prevent the aforementioned destructive overspeed condition for the fluid drive unit.